1. Field of the Invention
The present invention relates to a semiconductor power conversion system which is used for AC-to-DC conversion, DC power transmission, frequency conversion, etc. in the fields of electric power systems, electric railways, large-scaled plants, etc. More particularly, it relates to a semiconductor power conversion system which is well suited for, e.g., a variable speed generator/motor that needs to continue running perfectly even in a case where an AC input voltage has fluctuated due to an abnormality of the electric power system or a case where the load of a DC output side is an inductive circuit, so an overvoltage develops due to a current source operation.
2. Description of the Prior Art
In a case where a conventional semiconductor power conversion apparatus of the line commutation type is applied to equipment with the intention of enhancing the stability of an electric power system, there is a problem as stated below:
Since the thermal capacities of semiconductor devices for electric power, such as thyristors, are smaller than those of transformers, rotary electric machines etc., the protective operation in the event of any abnormal accruing is inevitably designed to be highly sensitive. With the semiconductor power conversion apparatus, therefore, the operation thereof must sometimes be interrupted in consideration of the protection of the power semiconductor devices even when the disturbance of the electric power system side is slight. Herein, once the operation of the apparatus of this type has been interrupted, it cannot be restarted until various items are inspected and confirmed to be secure and safe. Accordingly, the time period for the interruption becomes long, and the apparatus cannot satisfactorily contribute to the enhancement of the stability of the electric power system.
A prior-art apparatus for solving such a problem is disclosed in the official gazette of Japanese Patent Application Laid-open No. 52699/1988.
FIG. 10 is a connection diagram showing the circuit arrangement of the prior-art example. The illustrated semiconductor power conversion apparatus is so constructed that electric power is fed from an AC system 1 to a three phase bridge circuit 3 through a power transformer 2.
Referring to the figure, the apparatus includes current transformers 5 for detecting three-phase AC input current values, a DC current transformer 6 for detecting a DC side output current value, and an input current detector circuit 7.
Besides, it includes a subtractor 6a for finding the difference between the output lac of the input current detecting circuit 7 and the output Idc of the DC current transformer 6, an absolute value calculating circuit 8 for finding the absolute value of the output of the subtractor 6a, a differential current detector 9, an overcurrent detector 10, and an operation continuation deciding unit 11.
The output Iac of the input current detecting circuit 7 representing an input current value and the output current value Idc from the DC current transformer 6 are equal during the steady operation of the conversion apparatus. A signal .DELTA.I obtained by rectifying the difference of both the current values by means of the absolute value calculating circuit 8, is applied to the differential current detector 9, while the input current value Iac is applied to the overcurrent detector 10.
The operation continuation deciding unit 11 has logic functions as indicated in detail in FIG. 12.
Referring to FIG. 12, if the output signal OC of the overcurrent detector 10 is "0" (step 100), the output current of the three-phase bridge circuit 3 is decided to be normal (step 104), and an operation continuation command G01 is output (step 107) so as to continue the steady operation.
However, even when the output signal OC of the overcurrent detector 10 is "0", processing for a return to the normal operation of the conversion apparatus (step 109) is required in the case where the normal operation is to be restarted after the return of the apparatus from the output of a suppression control command G02. To this end, it must be decided whether or not the overcurrent suppression control command G02 was recently delivered (step 108). Subject to the delivery of the command G02, the return processing (step 109) is executed, and the operation continuation command G01 is thereafter outputted (step 107).
Here, the contents of the return processing (step 109) are as follows: i) In the state of the conversion apparatus before the detection of the overcurrent, some of the calculated results of a control unit (not shown) for controlling the gates of the individual thyristors of the three-phase bridge circuit 3 are initialized on the basis of circuit currents at present. ii) The present firing conditions of forward and backward thyristors TYS are sensed by the control unit, and these thyristors are initialized.
On the other hand, if the output signal OC of the overcurrent detector 10 is "1" (step 100), the value of the output signal .DELTA.I of the absolute value calculating circuit 8 is further decided (step 101). If the signal .DELTA.I is below an allowable value K1 and nothing is wrong with a generator, then the overcurrent is decided to be attributable to a power system fault or an interrupted operation in another power conversion apparatus (steps 110, 103), and the overcurrent suppression control command G02 is output to the control unit (step 106).
The thyristor short-circuiting switch TYS is fired in compliance with the overcurrent suppression control command G02, thereby suppressing the current of the three-phase bridge circuit 3. Incidentally, the output of the suppression control command G02 is recorded by a flip-flop circuit (not shown) or the like in order to prepare for the processing required when returning to the normal operation (step 109).
Besides, in a case where the value of the output signal .about.I of the absolute value calculating circuit 8 is equal to or above the allowable value K1 (step 101), the overcurrent is decided to be attributable to an internal fault of the three-phase bridge circuit 3 (step 102), and an operation emergency stop command ST is output (step 105).
The firing signals of the thyristors TY1-TY6 of the three-phase bridge circuit 3 are forcibly blocked in compliance with the operation emergency stop command ST.
Here, even in case of an internal fault in another piece of equipment, the signal .DELTA.I might fall below the allowable value K1. Therefore, whether such an internal fault is present is decided upon after the decision of the step 101 and step 110. In the presence of the internal fault of the other equipment (step 111), the operation emergency stop command ST is outputted in the same manner as in the case of the internal fault of the power conversion apparatus (step 105) without carrying out the overcurrent suppression control.
With the prior-art technique, in detecting the abnormality of the power conversion apparat is not satisfactorily made between an abnormality attributable to the fault or damage in any equipment and an abnormality, such as commutation failure or overvoltage, caused under the influence of a fluctuation on the AC system side or the DC side. For example, even in the abnormality of the commutation failure attributable to the external fluctuation, the conversion apparatus is subjected to an emergency stop when the overcurrent signal OC has become "1" with the signal .DELTA.I reaching the allowable value K1. This causes the problem that the operating reliability of the conversion apparatus cannot be ensured.